They have the mamoth tank that can take a lot of damage. Try destroying a mamoth tank with a NOD stealth tank or any other NOD tank, its not an easy task.

The GDI infantry is far more superior than NODs infantry. Hell a simple infantry soldier was able to destroy a stealth tank with onky a few shots.

The GDI Advanced guard tower is better than the NODs laser tower. Sure it make have 2 turrets but its not like the advanced GDI guard tower. You can simply avoids NODs laser tower by jumping every few seconds. Where as the GDI tower has both rockets and machine guns... try avoiding those while trying to sneak into the base.

And NODs stealth tank is not really all that stealth now its it. Hell infantry soldiers can spot the tank. And even the NOD infantry soldier that can go stealth is the same thing, you can always get spotted.

Personally I think the sides are not matched evenly. GDI has a far more advantage than NOD. Thats why everyone picks GDI